In the Launchpad Bug reports https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1005699, see item #24 for the fix. The discussion talks about a tigon3 driver problem in the kernel. The 3.0.24 and below tree had this error but as Ubuntu now has support for a 3.0.25-40 kernel the bug has been fixed. Item #20 talks the details of how this problem was discovered by working back from the development kernels 3.4, 3.3, and 3.2 and how it was discovered what the tg3_stop_block_error means. It is a problem with TSO. TSO stands for TCP Segmentation offload.

What happened in the pre 3.0.25 kernel is explained by Matt Carlson here

On the earliest TSO capable devices, TSO was accomplished through
firmware. The TSO cannot coexist with ASF management firmware though.
The tg3 driver determines whether or not ASF is enabled by calling
tg3_get_eeprom_hw_cfg(), which checks a particular bit of NIC memory.
Commit dabc5c670d3f86d15ee4f42ab38ec5bd2682487d, entitled "tg3: Move
TSO_CAPABLE assignment", accidentally moved the code that determines
TSO capabilities earlier than the call to tg3_get_eeprom_hw_cfg(). As a
consequence, the driver was attempting to determine TSO capabilities
before it had all the data it needed to make the decision. This patch fixes the problem by revisiting and reevaluating the decision
after tg3_get_eeprom_hw_cfg() is called.